The subject matter of the invention is a process for protecting stranded cable conductors in aggressive surroundings. Stranded cable conductors, which comprise a plurality of individual metal wires braided or twisted with one another, are often used as electrical lines for connecting electrical components.
Many electrical components are nowadays arranged in aggressive surroundings. For example, in motor vehicles fuel pumps for delivering fuel are used directly in fuel tanks. During the operation of the fuel pump, the fuel also flows through the electric motor driving the pump, for cooling purposes. Consequently, fuel also washes around the connecting joints of the electric motor with the electrical lines that are present. These connecting joints are attacked by aggressive constituents in fuels in such a way that the connecting joints are damaged and the component ultimately fails. However, it is not only the constituents contained in fuels that lead to damage. Similarly, moisture, dirt or rainwater can lead to stranded cable conductors being damaged at other points.
It is known in this respect to surround stranded cable conductors with insulating sheathings. What is disadvantageous about sheathings of this kind is that, when the stranded cable conductors are connected to a component, this sheathing has to be removed and the connecting joint is subsequently exposed. For protecting the connecting joints, it is known to provide them with a protective coating. The disadvantage of protective coatings that are used is that they are not resistant to all aggressive media, especially ethanol, which is a constituent of some fuels. What is more, the use of protective coatings is disadvantageous from ecological aspects.
It is known furthermore to insulate stranded cable conductors with shrinkable tubes, in that a tube of an appropriate plastic is shrunk on under the effect of heat. In addition to insulation, shrinkable tubes do indeed also offer a certain protection against aggressive media. However, the connection between the shrinkable tube and the stranded cable conductor is never impermeable enough for penetration of the aggressive media between the stranded cable conductor and the shrinkable tube to be reliably prevented, so that damage to the stranded cable conductor is not ruled out. Furthermore, in particular in the case of small components, the assembly of stranded cable conductors insulated in this way entails difficulties on account of the elasticity of the shrinkable tube. A further disadvantage is that shrinkable tubes cannot be used unrestrictedly. For instance, a shrinkable tube can only be used for protection of a connecting joint between a stranded cable conductor and an electrical component if the component and the stranded cable conductor have approximately the same dimensions with regard to the diameter.
The invention is based on the object of providing a process for protecting stranded cable conductors to protect them effectively against the attack of aggressive media. The connecting joints between stranded cable conductors and electrical components are especially to be protected by the process. The object is achieved by the features of claim 1. Further advantageous developments are described in the subclaims.
It has been found that stranded cable conductors can be effectively protected in aggressive surroundings by being partially encapsulated in a fuel-resistant plastic. This means that the stranded cable conductor is not encapsulated over its entire length but only in the region in which the stranded cable conductor is exposed to aggressive media.
This is all the more surprising since prevailing opinion assumes that the plastic under high pressure during injection molding will penetrate between the wires of the stranded cable conductor and, after hardening, form sharp edges in the outer region of the encapsulation in particular. If the stranded cable conductor is then subjected to movements, as occur for example in a motor vehicle, these movements may lead to the individual wires being severed at the edges. As a consequence, the stranded cable conductor may break and the electrical component may ultimately fail.
However, with the process according to the invention, the feared penetration of the plastic between the wires of the stranded cable conductor does not occur, so that from now on stranded cable conductors may also have partial encapsulations of plastic without the risk of the stranded cable conductor being damaged at the edges of the plastic in which it is encapsulated when there are movements.
The great advantage is that encapsulations can be produced in a wide variety of forms. The protection against aggressive media is consequently no longer restricted exclusively to the simple form of the stranded cable conductor. Rather, complicated geometries can also be encapsulated, so that from now on the connecting joints of the stranded cable conductor and the electrical component in particular are effectively protected against attack by aggressive media and the stranded cable conductor does not need any prior treatment.
Nevertheless, production defects or previous assembly may have the effect that the stranded cable conductor no longer has the necessary impermeability. In these cases, it is advantageous for the stranded cable conductor to be mechanically compressed in the outer region of the encapsulation. As a result, the penetration of plastic into the stranded cable conductor is prevented.
In another advantageous development of the process, before being encapsulated in plastic, the stranded cable conductor is surrounded in the region of the ends of the encapsulation with a thin sleeve of a fuel-resistant metal. This sleeve prevents penetration of the plastic even more reliably during encapsulation of the stranded cable conductor. At the same time, the pressure during the process ensures the necessary sealing between the stranded cable conductor and the sleeve, so that no fuel can penetrate between the sleeve and the stranded cable conductor.
In other applications, in which two components are electrically connected to a stranded cable conductor, it may happen that the components are already adequately protected and only the stranded cable conductor is in an area with aggressive media. In such cases, the process according to the invention can likewise be used for providing the stranded cable conductor with the partial encapsulation, here again the encapsulation being restricted to the region at risk.
In a further advantageous development of the invention, in which the electrical component is arranged in a housing and the stranded cable conductor is led out of the housing, the partial encapsulation of the stranded cable conductor serves at the same time as a seal between the stranded cable conductor and the housing.
In another advantageous development of the process, polyoxymethylene (POM) or PPS is used as the plastic. The use of these fuels [sic] has the advantage that these plastics is [sic] resistant not only to fuel but also to other aggressive media.
A further advantage is that, by suitable choice of the wall thickness of the plastic, the mobility of the stranded cable conductor can be specifically set. Even with small wall thicknesses, the mobility is restricted to the extent that, on account of the mechanical rigidity, the stranded cable conductor is self-supporting in this region. Similarly, the encapsulation of the stranded cable conductor may enforce a certain form, which corresponds to the stranded cable conductor in the assembled state. As a result, assembly can be simplified significantly.